1. Field of the Invention
This invention relates to a fuel metering system having improved ability to handle transient fuel metering modes of operation. More particularly, it relates to a fuel metering system for an internal combustion engine wherein the fuel control system of the engine is better enabled, as compared to the prior art, to handle the transient conditions that occur during engine accelerations, decelerations (negative acceleration) and other conditions that cause fluctuations to occur on a temporary basis in the flow of fuel from the engine's primary fuel metering apparatus to its combustion chamber or chambers.
2. Prior Art
In internal combustion engines, the rate at which fuel is metered to the engine varies during engine operation. Changes in engine load cause the engine's fuel metering apparatus to increase or to decrease the rate at which fuel is metered to the engine. As a result, the engine must change from a first state, where the engine operation and fuel flow rate is quite stable to a second state, where these conditions again become stable. The conditions in between the stable states are of a transient character in that the rate of fuel flow varies continuously and can produce undesirable air/fuel ratios. For example, with carburetion or other central location of the fuel metering apparatus, there is an intake manifold passage that the vaporized or atomized fuel must traverse in order to reach the engine's combustion chamber or chambers. At a given engine load, prior art fuel control systems under transient engine operation are unable to maintain precise air/fuel ratios until the conditions in the engine's intake passages have stabilized. Sudden accelerations create a need to deposit liquid fuel on the walls of the intake passages (wall wetting), while sudden decelerations result in the evaporation of the previously deposited fuel. The reason for this has to do with the changing vapor pressures. The higher the vapor pressure, the more the fuel tends to accumulate on the walls of the intake passages. Vapor pressure is a partial pressure, and the major contributor to pressure in the intake pressure is air. The air pressure in the intake passages in general is below atmospheric, unless the usual throttle valve is fully open, during engine operation.
While the wall-wetting changes, the amount of fuel metered by the fuel metering apparatus on the engine is not the amount of fuel that actually reaches the engine's combustion chambers within the charge transport time (air/fuel delivery time) applicable to the particular engine speed and load conditions at the time. The engine speed and load under stable engine operating conditions are the factors primarily determinative of the transport time of the air/fuel mixture from the fuel metering apparatus, such as a conventional carburetion system, to the engine's respective combustion chambers.
When the engine is cold, the amount of liquid fuel deposited on the intake passage surfaces is greater than it is when the engine is warm. This is because the tendency to vaporize fuel is greater at higher temperatures, and also because the fuel condenses more easily at the lower temperatures. Also, at lower intake air or fuel temperatures, the fuel metering device or system employed may not be as effective in thoroughly mixing the air and fuel inducted into the engine. For these reasons, it conventionally has been necessary to employ fuel enrichment devices and techniques (the general equivalent of the choke function conventionally employed on spark ignition engines) in order to compensate for operation at lower temperatures. Unfortunately, the fuel enrichment that occurs results in increased hydrocarbon engine exhaust emissions and this has necessitated the use of elaborate choke control devices and systems to reduce the hydrocarbon emissions as much and as rapidly as possible. Such reduction of the hydrocarbon emissions has impeded or reduced the performance of the associated engines during the warm-up period.